by Ben Mutzabaugh, USA TODAY

The carrier's first arrival was United Flight 958, a Boeing 767 arriving from Chicago O'Hare to Terminal 2's gate 38B at 5:49 a.m. local time. The flight is the first of 17 arrivals and the same number of departures that are scheduled for United on Wednesday at Terminal 2.

United says the Terminal 2 flights bring all of its operations at Heathrow - previously split between Terminals 1 and 4 - "under one roof" for the first time.

"We're extremely proud to be the first airline to operate from the new Terminal 2 and to consolidate our Heathrow operations in the airport's most advanced terminal," Bob Schumacher, United's managing director of sales, U.K. & Ireland, says in a statement. "Heathrow is one of the most important airports in United's global network, and Terminal 2 represents a huge improvement in the service and facilities we are able to provide to all our customers, whether they're departing, arriving or connecting."

The new terminal is designed to serve 20 million passengers a year and be the home of 23 Star Alliance airlines as well as Aer Lingus and a few other carriers. The opening will be in phases, with United Airlines starting its flights to T2 on June 4 and others moving in during the following six months.

The BBC says the phased-in opening is in part to "avoid the chaos in 2008 when Terminal 5 opened and staff struggled to cope with the computer and baggage systems."

"The new terminal will operate at 10% of capacity on the first day and it will be some months before the other 25 airlines using Terminals 1, 3 and 4 join United Airlines at the new building," the BBC adds.